1. Field of the Invention
The present invention relates to an electric power steering apparatus which gives a steering assist force to a steering system of an automobile or a vehicle by driving a brush-less DC motor.
Also, the invention relates to a controller for the apparatus, and more particularly to a high-performance electric power steering apparatus in which a cogging torque compensation value is added to a d-axis current command value to reduce the cogging torque of a motor, and also to a controller for the apparatus.
In addition, the invention also relates to a controller for the apparatus, and more particularly to an electric power steering apparatus in which a friction compensation flux-weakening current that is a friction compensation value is added to a d-axis current command value to reduce friction of a steering, and also to a controller for the apparatus.
2. Description of the Background Art
An electric power steering apparatus which auxiliary-load-energizes (assists) a steering apparatus of an automobile or a vehicle by means of a torque of a motor is configured so that a steering shaft or a rack shaft is auxiliary-load-energized with the driving force of the motor by a transmission mechanism such as gears and a belt via a reducer. Such a conventional electric power steering apparatus performs a feedback control of a motor current in order to correctly generate an assist torque (auxiliary steering force). In the feedback control, a voltage to be applied to the motor is adjusted so as to decrease the difference between a current command value and a motor current detection value. The motor-applied voltage is usually regulated by adjusting the duty ratio of a PWM (Pulse-Width Modulation) control.
A usual configuration of an electric power steering apparatus will be described with reference to FIG. 6. A column shaft 2 of a steering wheel 1 is coupled to a tie rod 6 of a steerable wheel through a reduction gear 3, universal joints 4A and 4B, and a rack and pinion mechanism 5. A torque sensor 10 which detects the steering torque of the steering wheel 1 is disposed on the column shaft 2. A motor 20 which assists the steering force of the steering wheel 1 is coupled to the column shaft 2 via the reduction gear 3. An electric power is supplied from a battery 14 to a control unit 30 which controls the power steering apparatus, and also an ignition key signal is supplied from an ignition key 11 to the control unit. Based on a steering torque value T detected by the torque sensor 10, and a vehicle speed V detected by a vehicle speed sensor 12, the control unit 30 calculates a steering assist command value I for an assist command with using an assist map, and controls the current to be supplied to the motor 20 based on the calculated steering assist command value I.
The control unit 30 is configured mainly by a CPU (or MPU or MCU). FIG. 7 shows usual functions to be executed by programs in the CPU.
Referring to FIG. 7, functions and operations of the control unit 30 will be described. The steering torque T detected by the torque sensor 10, and the vehicle speed V from the vehicle speed sensor 12 are supplied to a steering assist command value calculating portion 31, and a basic steering assist command value Iref1 is calculated. The calculated basic steering assist command value Iref1 is subjected to phase compensation by a phase compensating portion 32 in order to enhance the stability of the steering system, and a phase-compensated steering assist command value Iref2 is supplied to an adder 33. The steering torque T is supplied also to a differential compensating portion 35 of a feedforward system which is used for enhancing the response speed. A differential-compensated steering torque TS is supplied to the adder 33. The adder 33 adds the steering assist command value Iref2 and the steering torque TS together, and supplies an steering assist command value Iref3 which is the result of the addition, to a subtracting portion 34.
The subtracting portion 34 obtains a deviation (Iref3-i) between the steering assist command value Iref3 and the fed-back motor current i. The deviation (Iref3-i) is PI-controlled in a PI controlling portion 36, and then supplied to a PWM controlling portion 37 to calculate the duty, so that the motor 20 is PWM-driven through an inverter 38. The motor current i of the motor 20 is detected by motor current detecting means (not shown), and then supplied to the subtracting portion 34 to be fed back.
In the thus configured electric power steering apparatus, as the motor 20 which is a driving source, a DC motor or a brush-less DC motor is used. A brush-less DC motor has three or more exciting phases, and is driven by a rectangular or sinusoidal exciting current supplied from the control unit.
In an electric power steering apparatus comprising such a DC motor, in order to improve the steering feeling of the driver, it is preferable not to generate cogging torque (pulsation torque) of the motor.
In an electric power steering apparatus disclosed in Japanese Patent Unexamined Publication JP-A-2005-67359, a correction value which offsets the pulsation torque component of a motor is superimposed on a target current. However, the target current value which is output as torque from the motor is corrected, and hence the correction value and cogging torque which are not offset because of an error are produced as vibration.
In a permanent magnet brush-less motor disclosed in Japanese Patent Unexamined Publication JP-A-2003-250254, the pole and slot numbers at which cogging torque of a reduced degree is produced are set. In this configuration, however, the degree of freedom in the deign of the motor is reduced.
As described above, in a conventional controller, cogging torque of a motor is not considered. Even in the case where a vehicle travels at a high or medium speed and a steering wheel is slowly rotated, i.e., in the case where the driver perceptively feels the steering feeling, when the motor control is performed in the same manner as other cases, the cogging torque of the motor is transmitted as vibrations to the steering wheel, thereby causing a problem in that the steering feeling is impaired. When a control in which the control gain is reduced so as to always eliminate influences of high-frequency noises and the like, or which involves complex calculations for eliminating pulsation torque is performed, there arises a problem in that the follow-up property of the motor with respect to a control command value is impaired.
Also in the case where cogging torque of a motor is corrected as in JP-A-2005-67359, when correction is performed on the target current value which is output as torque from the motor, the correction value and pulsation torque (cogging torque) which are not offset because of an error are transmitted as a torque ripple from the steering wheel to the driver.
Also in the case where cogging torque is reduced by the structure of a motor as in Japanese Patent Unexamined Publication JP-A-2003-250254, there is a problem in that the degree of freedom in the deign of the motor is largely reduced, for example, a motor which is frequently used, and in which the ratio of the pole number to the slot number is 2:3 is excluded.
Furthermore, in an electric power steering apparatus comprising such a DC motor as shown in FIGS. 6 and 7, it is requested to improve the on-center feeling of the steering wheel. However, the torque loop response in the vicinity of the center is insufficient, and hence mechanical or electromagnetic loss torque must be compensated. Also when the steering wheel is caused to return by self-aligning torque (SAT), mechanical or electromagnetic loss torque must be compensated in order to enable the steering wheel to return to the neutral position. In order to reduce the swing of the steering wheel to allow the steering wheel to rapidly return to the neutral position, damping must be applied to an electric power steering system in accordance with the rotational speed of the motor.
The loss torque is part of the output torque of the motor, and means a frictional loss based on the structure of the motor, and a loss which is produced by an electromagnetic cause. When a slight steering operation is performed in a straight traveling state, such a torque loss causes an unintended force to be applied in the steering direction, or a force in the opposite direction to be applied, thereby impairing the steering feeling. As a countermeasure against this, a method in which the current command value is increased by a degree corresponding to loss torque has been proposed. In the method, when a small drift is produced in a detection value of the steering torque, however, an auxiliary steering force is applied in the direction opposite to the steering direction, thereby causing a problem in that, when the steering wheel tries to return, the method does not operate effectively. This problem easily arises particularly when the steering operation is performed in the vicinity of the center in a high-speed traveling state.
In the technique disclosed in Japanese Patent Unexamined Publication JP-A-10-109655, attention is focused on the fact that loss torque is a function in the rotational direction of a motor, and a loss torque compensation value based on the motor rotational direction which is detected from an estimate value of the angular velocity of the motor is added to a current command value.
When the loss torque compensation or the damping application is to be performed, however, it is required to detect the rotational direction or rotational speed of the motor. When means for detecting such a value is disposed, the number of components of the electric power steering apparatus is increased. Furthermore, the detection accuracy of the means must satisfy the performance requested in the electric power steering apparatus. Therefore, the cost of the electric power steering apparatus is increased.
As a motor in which loss torque is reduced, Japanese Patent Unexamined Publication JP-A-2001-73095 discloses a PM (Permanent Magnet) motor in which loss torque of the motor is reduced by improving the content ratios of materials of a steel plate and the production method. However, the motor has a problem in that the material and production costs of the motor are increased.